There is an increasing demand for tunable lasers given the advent of wavelength-division multiplexing (WDM) which has become widespread in fiber optic communication systems. WDM transponders include a laser, modulator, a receiver and associated electronics. One WDM transponder operates a fixed laser in the near-infrared spectrum at around 1550 nm. A 176 wavelength system uses one laser per wavelength and, therefore, such a system typically must store a 176 additional WDM transponders as “spares” to deal with failures. The high inventory requirement contributes to the high cost of those systems.
In response, tunable lasers have been developed. A single tunable laser can serve as a back-up for multiple channels or wavelengths so that fewer WDM transponders need to be stocked for spare parts. Tunable lasers can also provide flexibility at multiplexing locations, where wavelengths can be added and dropped from fibers as needed. Accordingly, tunable lasers can help carriers effectively manage wavelengths throughout a fiber optics network.
The currently available tunable lasers are distributed feedback (DFB) lasers and distributed Bragg reflector (DBR) lasers. A conventional prior art tunable laser module 1 is illustrated in FIG. 1. In tunable lasers, the output power is most often measured from the front of gain medium 2 of laser device 1, and not from a rear facet (as is done with non-tunable lasers). The output of gain medium 2 is directed through a collimating lens 3 and isolator 4. The optical output then engages a power tap 5 at an angle of about 45°, so that a fraction of the signal is directed toward a detector 6; the remainder of the output passes through lens 7 and into an optical fiber 8. The output from detector 6 is applied to a tuning element 9, which controls the operation of gain medium 2 (for example, through temperature adjustments) to tune the wavelength of laser 1.
One difficulty with many of the conventional prior art tunable lasers, however, is that they require a number of discrete components, these components each having a number of surfaces that may introduce stray, unwanted reflections into the laser, disturbing the ability to “tune” the laser over the desired wavelength range.
As with most communication systems, the efficient use of space and power in optical transmitters (e.g., lasers) is of ever-increasing importance. Further, design considerations for these transmitters must take into account the modularity of the particular components that are included in the network.
It would be desirable, therefore, to configure a tunable laser module which is relatively small in size, yet retains the wide range of tunability needed for many of the desirable WDM applications.